1. Field of the Invention
The present invention relates generally to the field of reaction bonded silicon nitride (RBSN) and more specifically to a continuous process for manufacturing articles made of RBSN.
2. Background Art
RBSN is conventionally made by heating a compact of silicon powder in a nitrogen gas atmosphere. Typically, the nitriding cycle is carried out at a temperature of about 1400° C. and can take up to two hundred hours or longer. Reaction bonding enables the production of complex near net shapes. Although silicon expands by about 20% on conversion to nitride, the expansion is accommodated by the initially void space of the silicon compact. Consequently, there is very little change in the volume of the articles. As a result, relatively accurate shapes and dimensions can be achieved while obviating expensive finishing tasks such as diamond grinding and the like. To the extent that the nitrided article needs further densification or shaping, hot pressing may be used to achieve the desired result. An example of prior art methods for producing RBSN articles may be obtained in issued U.S. Pat. Nos. 4,235,857 to Mangels; 4,848,984 to Ezis et al; and 4,946,630 to Ezis, all of which are assigned to the assignee hereof and each of which disclosure is incorporated herein by reference.
Conventional prior art RBSN fabrication processes have various disadvantages which are addressed by the present invention. First and foremost among these disadvantages is the processing time required to produce finished parts. Usually, the most significant part of the overall processing time is the number of hours required for the nitriding process. As mentioned above, this number is typically as much as 200 or more hours when starting with a green compact part.
Another concern associated with conventional RBSN processing is thermal runaway. As Si reacts with N2 and becomes Si3N4, waste heat is generated within the furnace. This heat is radiated to other nearby parts, which prompts more reaction, which generates more heat, which makes the reaction speed up. Such thermal runaway can create a temperature that is sufficiently high to melt the silicon which becomes resistant to nitriding thereby compromising the resulting material. This problem is exacerbated when the parts are crowded together within the furnace where the radiated heat of each part's reaction can reach and affect nearby parts.
Still another concern is the total amount of energy that is required to nitride the silicon compacts over a long period of time at high temperatures. Running a nitriding furnace is an expensive use of electrical energy, particularly at today's high cost of electricity. It takes about 21,000 Kwh to run a conventional nitriding furnace per batch of product. At current rates of about 14 cents/Kwh, each such furnace costs almost $3,000 in energy alone to run one batch of product.
Yet another concern is cost of maintenance. The constant temperature cycling of each furnace and the exposure to air during loading and unloading of each product batch, requires frequent and costly maintenance that is labor intensive and interrupts part production.
Therefore, it would be highly advantageous if it were possible to reduce processing time, eliminate thermal runaway, reduce energy costs and maintenance in the fabrication of RBSN parts.